When someone leads you to believe that you have won a hand in poker, but then turns over a very strong hand.

"I was offended at the way Brad slowrolled me," Jim said.
"What did he do, exactly?" Joe said.
"At the showdown when I turned over my cards, he didn't turns his up after seeing my hand. While I was collecting the chips, he turned over the nuts."

When a person seeking an answer is perpetually put off, usually by having to answer subsequent questions given consecutively, before his original question may be answered.

Before leaving, Fred's boss Allen executed a perfect slow roll maneuver, delaying Fred's departure by asking made up questions, the answers of which Fred had to find before Allen would allow Fred to depart.